A video surfacing on social media shows a man hopping over a barrier, reaching down, and quickly slapping one of two hippos' behinds.
He then quickly gets out and appears to give a thumbs up as he flees.
It's unclear when the incident happened, but officials said it was a big risk because hippos kill more people than lions in Africa.
The two hippos, mother Rosie and daughter Mara, were the ones in the enclosure.
Zoo staff also said it was a potentially dangerous incident not just for the man, but the mom and daughter hippo.